Following the drilling of wells such as oil or gas wells, it is desirable to characterize the formations surrounding the well in order that the appropriate measures can be taken to obtain optimum production from the well without causing damage to the formations and preventing effective recovery of the useful reserves. Properties such as permeability (horizontal, spherical, vertical, etc.), skin (the extent of damage to the formation around the well, typically arising from the drilling and well completion processes), and the limits of the producing formations are some of the properties typically used for such characterization. One tool used to make measurements to allow these properties to be determined is the MDT tool of Schlumberger. The MDT tool includes a highly accurate pressure gauge, the CQG, to enable measurements to be made of the fluid pressure in the formation around the well or in isolated intervals of the well. One well test that is possible with such a tool is the mini-DST in which fluid is drawn from an interval of the well isolated by means of a pair of inflatable packers and the pressure in the interval monitored as fluids flow from the formation into the interval. The pressure difference is measured over time and the pressure derivative, the rate of change of pressure over time determined. From these measurements, the properties of the formations can be determined. Examples of mini-DSTs can be found in The MDT Tool: A Wireline Testing Breakthrough, Schlumberger Oilfield Review, April 1992, 58-65; and Characterizing Permeability with Formation Testers, Schlumberger Oilfield Review, Autumn 2001, 2-23.
The fundamental challenge in downhole pressure gauge design is to provide something which is mechanically very strong so as to be able to withstand high static and dynamic stresses, but at the same very sensitive to small pressure changes (for accuracy and resolution), which implies mechanically weak. In high-end gauges such as CQG, there can be a dynamic range of more than 106 between the gauge pressure rating (e.g. 137895 kPa (20000 psi)) and the gauge resolution (e.g. 69Pa (0.01 psi)). Sensors with such a large dynamic range are difficult to design and manufacture.
However, if the way in which the data is used is considered, high resolution is not normally needed at the same time as high accuracy. For example, the resolution is used at the end of build-ups in well testing but there is no need to determine reservoir pressure to within 69 Pa (0.01 psi). The high resolution measurement is used to compute (and process and interpret) the derivative of the pressure signal.
This invention is based on the use of a pressure sensor that directly measures the derivative of the pressure. This can be achieved much more simply and at much lower cost than conventional high end pressure sensors.
A differential pressure gauge is described in A Deep-Sea Differential Pressure Gauge, Cox, C. et al, Journal of Atmospheric and Oceanic Technology Vol. 1, September 1984, 237-246. This document proposes a pressure sensor for use in sea-bed conditions to measure pressures generated by long ocean surface gravity waves, seismic disturbances of the seabed, microseisms, and the low-frequency end of the ocean acoustic spectrum, at frequencies in the range 102-10−4 Hz and at pressures of 105-10−5 Pa2/Hz, with a seabed pressure of 4×107 Pa.
U.S. Pat. No. 4,507,971 describes an apparatus for measuring pressure. The apparatus comprises a hydraulic filter having a capillary tube as a resistor to eliminate slow pressure fluctuations.